The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, nitrogen oxide (NOx) included in exhaust gas is a cause of acid rain, harms eyes and the respiratory organs, and withers plants. NOx is regulated as a major air pollutant and research has been carried out in order to reduce the amount of NOx in exhaust gases.
An exhaust gas recirculation (EGR) system mounted in a vehicle reduces noxious exhaust gases of the vehicle. Generally, the amount of NOx in the exhaust gas is increased in an oxygen rich air mixture, and the air mixture is combusted well. Therefore, the exhaust gas recirculation system reduces the amount of NOx in the exhaust gas as a consequence of a part (e.g., approximately 5-20%) of the exhaust gas being recirculated to the air mixture in order to reduce the oxygen ratio in the air mixture and so hinder combustion.
Generally, an exhaust gas recirculation system includes a hot EGR system that reduces the amounts of NOx, particulate material (PM), and hydrocarbon (HO) exhausted by retarding an ignition and reducing an air/fuel ratio, and a cooled EGR system that lowers a temperature of a combustion chamber and hinders combustion by cooling a recirculated exhaust gas.
An EGR ratio means a ratio of an EGR gas amount supplied to the engine through an exhaust gas recirculation system in an entire air amount (fresh air+EGR gas) supplied to the engine. When the EGR ratio is increased, the oxygen amount supplied to a combustion chamber is decreased, combustion temperature is decreased, and nitrogen oxide is decreased. But when the EGR gas amount is excessively increased, engine output and fuel consumption are deteriorated. However, when the EGR ratio is decreased, the oxygen amount supplied to a combustion chamber is increased, combustion temperature is increased, and nitrogen oxide is increased. Therefore, it is desirable to control the EGR gas amount.
Further, we have found that some EGR gas remains in an intake line in which fresh air and the EGR gas flow after the engine is turned off. The remaining EGR gas flows into a combustion chamber of the engine when the engine is turned on, which can result in an abnormal combustion, such as a knocking and pre-ignition.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.